The invention is particularly applicable to glass melting tanks of the size and capacity used in the manufacture of molten glass to be fed to a flat glass forming process such as the float process. Such tanks provide glass for forming on a float bath at a rate sufficient to produce as much as 6000 tonnes and not less than 500 tonnes of finished saleable glass per week. In such tanks solid batch material for forming the glass is normally fed into a melting zone of a glass melting tank where heat is applied to melt the batch material. The melting zone forms an upstream cell and this is normally followed by a downstream cell in which refining and conditioning of the glass occur before the glass is fed through an outlet to a glass forming process. This has been described in our UK Patent Specification No. 1503145. There are problems in providing homogeneous glass free from undissolved solios and gases particularly at high tank loads when time and temperature in particular zones are limited by furnace design and refractory constraints. Glass varying in composition forms layers in the furnace and these are subject to convective and other flows imposed by the furnace operation and other physical operations which may be carried out on the glass. In general, in the final product any inhomogeneity may exist as plates in the glass and where these are not parallel to the faces of flat glass which is subsequently formed, optical faults called ream may result. The object of refining and conditioning is to attempt to control bubble level homogeneity, ream, orientation of any inhomogeneity and temperature condition of the glass as it leaves the melting tank. Commonly the melting and refining regions of the tank are arranged to hold glass of such a depth that convection occurs resulting in forward and return flows of molten glass in both zones. It is also common practice to provide heat in the melting zone of such a glass melting tank by use of fossil fuels used to fire burners located adacent ports in the melting zone located above the level of the batch material which may form a blanket on top of the molten glass in the melting region. Such burners may in some cases be assisted by electrical heating provided by electrodes mounted in the melting zone in the molten glass. In such a glass melting tank, the region where the melting zone joins the refining zone may form a so-called "hot-spot" in which the glass is at its highest temperature and forms an uprise region in which glass tends to flow from the bottom of the tank towards the surface of the molten glass. This may cause convection flows resulting in forward flowing glass moving through the refining zone towards the outlet end with a proportion of the glass forming a return flow through the refining zone which passes in contact with the base of the refining zone and subsequently rises again at the hot-spot for recycling through the refining zone. Similarly some glass rising at the hot spot will tend to form a return flow in the melting zone in the upper regions of the molten glass immediately below the blanket of batch material and this will circulate to form a forward flow at the base of the melting zone where it may rise again at the hot-spot. The convection flows which result in such a case depend on the hot-spot temperature as well as the furnace load and the temperature gradient in the molten glass. The temperature at the hot-spot can be as high as 1500.degree. C. in the case of molten glass being prepared for use in a float forming process. Similarly the temperature at the bottom of the melting zone may be of the order of 1100.degree. C. to 1300.degree. C. In regions of forward and return flow the depth of glass in the tank may be in the order of 100 to 125 centimeters.
The provision of a relatively deep refining zone in which such circulation of glass due to convection currents occurs has been considered important in the production of high quality molten glass such that float glass may be produced with not more than 2 inclusions per kilogram wherein an inclusion is defined as any defect over 50 microns in size. The reason why the circulation within the refining zone has been important is that only a small portion of the newly melted glass leaves the furnace without being recirculated through the refining zone and this provides more time for removal of defects and reducing the overall level of occurrence of defects. Furthermore that portion of the forward flowing glass which flows from the refining zone towards the outlet end is conveyed through the refining zone on a roller of molten glass out of contact with the refractory at the base of the refining zone thereby avoiding contact with, and contamination by, the refractory base. The refining zone is inevitably a very hot region in order to allow satisfactory refining to occur. The return flow of glass in the refining zone which is in contact with the refractory at the base of the refining zone may develop defects due to interaction with the refractory but if this occurs it is returned to the hot-spot region which recirculates the glass at an increased temperature providing a further opportunity for the dissolution of solid inclusions and the escape of gaseous inclusions before the glass leaves the tank. In order to achieve satisfactory operation it is important that the position of the junction between the melting and refining regions is stabilised so that the effect of the circulation patterns within the melting and refining zones is predictable in achieving the required result in the glass which is finally supplied through the outlet of the glass melting tank. Any changes in thermal conditions which may alter the location of the hot-spot can have an adverse effect on the quality of the glass. For this reason various physical components have been used to improve the stability of the flow regime in such glass melting tanks. For instance, waists, weirs, floaters, water pipes, bubblers and other devices have been used to maintain an equilibrium state. In this way stability and high glass quality have been achieved. However these devices have the disadvantage of requiring fossil fuels for burners located above the batch material in the melting zone in order to cause the required circulation patterns within the melting zone and thermal inefficiency occurs due to the need to reheat the return flow of glass in the refining zone which is cooled and then reheated on rising at the hot-spot. Furthermore the furnace construction is large and the capital cost is high. Furthermore any changes from one glass composition to another such as may be needed to introduce a changing colour require protracted operation of the furnace in order to cause the gradual change in composition required.
In our UK Patent Specification No. 1533979 we have described a glass melting tank in which the area of the melting tank known as the conditioning zone is relatively shallow and in which there is no, or substantially no, return flow of molten glass away from the outlet end. In the case of a conditioning zone the glass is cooler than that in a refining zone as there is a progressive reduction in molten glass temperature on passing downstream from the hot-spot through the refining zone and through the conditioning zone to the outlet. There are problems in providing a relatively shallow refining zone in which all, or substantially all, the flow is towards the outlet end due to the high temperatures required in refining glass for feeding to a float process. In order to achieve a state of no return flow in the refining zone the depth of molten glass in the refining zone may be reduced to the order of 25 to 30 centimeters and the temperature used in refining soda lime glass may result in a glass/refractory interface at the bottom of the refining zone at a temperature of 1430.degree. C. to 1450.degree. C. This may be of the order of 200.degree. C. hotter than would be the case at the bottom of a refining zone where the glass is of 100 to 125 centimeters depth with return flow at the bottom of the refining zone. Generally, molten glass in contact with refractory at the bottom of the refining zone at temperatures as high as 1430.degree. C. to 1450.degree. C. tends to introduce defects such as bubbles of gas and particles of refractory.
The refractory material of the present invention has enabled this problem to be overcome. It has previously been proposed to provide a layer of molten metal such as tin at the base of a refining zone but there are formidable problems to be solved in containing and maintaining molten metal within the refining zone.